Continuation is proposed of a comprehensive, quantitative examination of fundamental aspects of mobility in old adults. Our overall hypothesis is that age, physical capabilities, multiple impairments, task demands, neuropsychological status and other psychological factors interact to determine abilities and willingness to perform physical activities of daily life and to alter risk of injury when performing those activities. We will study 1280 subjects in three Projects designed to test specific hypotheses concerning three postulated relationships. Project 1 (PI: Neil Alexander) will test quantitative models that predict the effects of multiple independent impairments on performance, and test the interactions of task complexity and impairment on performance measures. Project 2 (P.I: James Ashton-Miller) will develop and test a model of the way in which psychological factors mediate between physical and cognitive capacities and success in task performance. Project 3 (P.I. Albert Schultz) will test hypothesis regarding how physical capabilities and cognitive status affect abilities to maintain balance, or to restore that balance when a fall initiates, or to safely arrest that fall in balance cannot be restored. Program research will be fostered through three Cores: (A) Administrative, (B) Human Subjects/Data Analysis and (c) Neuropsychology. The test of investigators includes personnel with expertise in Biomechanics, Biostatistics, Experimental Psychology, Geriatric Medicine, Neuropsychology and Social Psychology. Understanding quantitatively how functional mobility is affected by aging, physical capabilities, neuropsychological status, other psychological factors and task performance biomechanical requirements will enable earlier detection and more precise assessment of declines in physical performance. It will show what factors are truly critical to abilities and willingness to safely perform daily physical activities, and thus lead to improved intervention in mobility impairments.